There has been known a stratified charge combustion enabling the combustion to take place by setting the air-fuel ratio in the whole cylinder to be leaner than the stoichiometric air-fuel ratio by vaporizing the fuel while it is flying after having been injected into the cylinder in the compression stroke to form a mixture in a portion of the cylinder, and by igniting the mixture using a spark plug to burn it. In the stratified charge combustion, a fuel injection valve injects the fuel in a manner that at least part of the fuel passes through the spark gap of the spark plug, the timing for ending the fuel injection is set to be close to the ignition timing, and the spark gap of the spark plug is positioned in the mixture at the ignition timing so that the mixture is ignited to burn.
In the stratified charge combustion, when fuel is required in a relatively small amount, the air-fuel ratio of the mixture that is formed becomes close to the stoichiometric air-fuel ratio i.e., a favorable stratified charge combustion can be realized. However, when fuel is required in an increased amount accompanying an increase in the engine load, the air-fuel ratio of the mixture that is formed becomes too rich making it difficult to realize a favorable stratified charge combustion and, besides, giving rise to a misfire.
To prevent this problem, there has been proposed a direct fuel injection-type spark ignition internal combustion engine according to which when the fuel is required in an increased amount, part of the fuel is injected in advance into the cylinder in the compression stroke to prevent the air-fuel ratio of the mixture from becoming too rich, the mixture being formed by the injection of fuel with its timing for ending the fuel injection set to be close to the ignition timing (see, for example, JP-A-2003-49679 and JP-A-2002-201990).
In the above direct fuel injection-type spark ignition internal combustion engine, if the amount of fuel injected by the first fuel injection near the ignition timing is suitably selected, then the mixture formed by the injection of fuel can be reliably ignited and burned. However, in case where the required amount of fuel is slightly larger than the amount of fuel injection suited for the first fuel injection and the amount of fuel injected by the second fuel injection before the first fuel injection is smaller than the amount of fuel injected by the first fuel injection, the pressure in the cylinder at the time of the second fuel injection is definitely lower than the pressure in the cylinder at the time of the first fuel injection. Therefore, the fuel injected by the second fuel injection is small in amount and is, further, subject to be dispersed. Therefore, the air-fuel ratio of the mixture formed by the injection of fuel becomes too lean.
It is difficult to ignite and burn the above too lean mixture even if it is caught by the burning mixture that is formed by the fuel injected by the first fuel injection. In the above case, therefore, it is difficult to realize the favorable stratified charge combustion. Much of the fuel injected by the second fuel injection is emitted as unburned fuel from the cylinder deteriorating the quality of emission and making it difficult to burn the required amount of fuel and, therefore, making it difficult to obtain a required engine output.
It is therefore an object of the present invention to provide a direct fuel injection-type spark ignition internal combustion engine which executes the stratified charge combustion by vaporizing the fuel injected into the cylinder while it is flying to form a mixture in a portion in the cylinder, and by igniting the mixture by using a spark plug to burn it, wherein a favorable stratified charge combustion is realized even when the fuel is required in a relatively increased amount.